Wednesday, 17 October 2018

Rule of Nine

Many moons ago, I had a burning interest in classic cars, many with British Motor Corporation 'A' Series engines. One of the useful tips I learned working on these was the Rule of Nine, a way to easily make sure your engine was in the correct position when adjusting valve clearances.

The thing is, when you adjust the clearance in a particular valve train you need to make sure the cam follower or tappet is on the base circle of the cam, or the clearance will be excessive. You do this by checking the clearance on both valves in a cylinder when that cylinder is at TDC. The thing is, wild valve timing or quietening ramps in the cam profile means you have to be accurate when positioning the engine.

But there is an easier way. Have a look at this camshaft:

That's a spare camshaft for my Square Four - a lot less worn than the one in the engine. On the right hand side of the picture, you see the thread retaining the cam chain sprocket, and that first cam is the exhaust cam for number one cylinder - notice it is pointing straight up and the valve will be fully open.

Here's a Stormy Petrel's eye view of the rockers, so you can see what I am talking about:

Move to the left hand end of the camshaft and we are looking at the last or eighth cam, the cam for the exhaust valve on number three. It's pointing straight down, which means the valve is fully closed and the tappet is on the base circle.

Now look at this picture. The cam that is pointing up is the inlet cam for number two cylinder - it's the fourth cam on the shaft.

The one pointing down is the fifth cam on the shaft, the inlet cam for number four cylinder, so

4 + 5 = 9

Remember the first example where we looked at the first cam and the eighth cam? Well:

1 + 8 = 9

Guess what? It works on the other valves too! So, if the valve on the second cam is fully open (which is very easy to discern) then you know that the valve on cam seven (the exhaust valve for number four) is on the base circle and can be adjusted.

Easy. The Rule of Nine.

By the way, I found that after 1000 miles, most of the clearances needed adjusting - maybe because I didn't do it very well last time.

Finish off with replacement of the rocker box gaskets with Threebond. The fibre gaskets specified by Ariel are not suitable for the sorts of temperatures the cylinder head sees.


  1. Hello Simes, thanks for this great guide, I've just done it with my SQ 4 MKII, has worked fine, the machine runs (felt) quieter and smoother.I Always had too much clearances may be with this quitening ramps.

    I have now looked at this process for my A7 / A10 and found that there it must be called

    "The rule of Five".

    It works just like that!

    Thanks again

    1. Excellent! I'm very pleased you found it useful. I found it much easier to set the clearances up using this method and my bike is quieter too.

  2. Thanks for that - I found it really interesting. I'm planning to re-commission my 1959 MkII square four this year. The rocker covers are a different design to those on your bike. Would Threebond still be suitable? I've never heard of it before. Several products are mentioned on their website. Would TB1102 be the correct product? (Liquid gasket, non-drying, excellent gasoline resistance for various flanges and threaded joints particularly those requiring frequent dis-assembly or adjustment). Thanks

    1. I actually use 1215 - it's a paste, and stays where you want it. It's also rated to 250 C. Mk2 rocker covers have a similar sealing requirement and I would use 1215 there too.